The present application is directed to inverter circuits used in the powering of discharge lamps, and more particularly to a third order high Q impedance matching inverter circuit with automatic line regulation electronic ballast for use with high power discharge lamps operating on a low input voltage.
Turning to FIG. 1, shown is a known, rapid start, second-order inverter circuit topology used for powering high power, low impedance discharge lamps. Such a circuit will have a 1 to 1.5 second delay between application of a starting signal and lamp ignition. Circuit 10 includes a full bridge input section 12 which receives an input from AC source 14. The output of the full bridge section 12 is provided to a half bridge switching circuit network 16, comprised of a first transistor switch 18, a second transistor switch 20, and a controller 21. Output voltage from the half bridge switching circuit 16 is delivered to a resonant LC network 22, including a resonant inductor 24 and a resonant capacitor 26. The output from LC circuit 22 is provided to a lamp 28, which is further connected to capacitive voltage divider network 30, composed of capacitor 32 and capacitor 34. A starting voltage of approximately 600 volts may be used as the ignition voltage. In this type of circuit, since the striking voltage is commonly only 600 volts, a preheat circuit (not shown) may be included to preheat the lamp prior to supplying the ignition voltage.
A drawback of the circuit depicted in FIG. 1 is that it is not designed to operate efficiently with high impedance lamps. This is due, in part, to the use of lower input voltage. For example, when the input is a standard 120 volts, the circuit bus voltage may be about 150-160 volts. The AC voltage is approximately halved, due to the operation of switching network 18, causing the AC output at the half-bridge switching network 18 to be approximately 75 volts. This voltage is sufficient to efficiently operate a low impedance lamp. However, if the lamp is a high impedance lamp, circuit 10 will need to draw an increased current, causing inefficient operation and stress on the components within the circuit.
Another drawback of the circuit in FIG. 1, is that in order to obtain an acceptable Q rating, if attempting to drive a high impedance lamp, a significantly higher voltage needs to be supplied to the lamp. In this situation, to obtain the desired Q rating, a larger sized resonant capacitor 26 and resonant inductor 24 is needed.
Further, the rapid start circuit 10 of FIG. 1, will maintain the preheat circuit active even after ignition of the lamp, resulting in a loss of about 1 to 1.5 watts of power.
If circuit 10 is attempted to be operated as an instant start lighting system, then the lamp starting voltage will be approximately 1300 volts. This higher voltage will need a higher resonant current, approximately 5 amps. The higher the current, the greater the stress on the inductor 24, requiring a larger sized component. Increasing the size of the magnetics (i.e., inductor 24) increases the cost of the magnetics, and increases the size of the housing in which the magnetics are held. The same switching current will also be seen by the half-bridge switching network 16, which includes transistors 18 and 20. To handle these higher currents, larger sized dies will be necessary, and therefore larger packages for transistors 18 and 20 will be used (the transistors may be FET, CMOS, bipolar or other appropriate transistor type). These larger, more robust transistors and capacitors carry an increased economic cost, require a larger physically sized lamp lighting system, as well resulting in decreased circuit efficiency.
Thus, if the second order inverter circuit 10 of FIG. 2 is attempted to be used to drive high impedance lamps, a large starting current would be needed. It is known that when the starting current is higher, larger magnetics (i.e., inductor 24), and transistors will be needed to handle the higher current, resulting in a less efficient lamp lighting system.
In accordance with one aspect of the present application, an inverter circuit includes an input section configured to receive voltage from a voltage source and to input the voltage to the circuit. A switching network is connected to receive the input voltage from the input section. A controller is placed in operational connection with the switching network and is designed to control operation of the switching network. A resonant switching circuit is configured to receive an output from the switching network. Load connections are connected to the resonant switching circuit. A variable capacitance network is connected to the load connection to provide a variable capacitance during circuit operation.
In accordance with another aspect of the present application, a method is provided for operating an inverter circuit, including supplying a voltage from a voltage source to an input section. The received voltage is passed from the input section to a switching network. Operation of the switching network is being controlled by a controller, wherein a prescribed voltage is transmitted to a resonant circuit and a lamp voltage is delivered to a lamp connected to the resonant circuit. A voltage in a capacitor is clamped at predetermined levels. The clamping action acts to remove a fixed capacitor from the circuit or at least a portion of a cycle of operation of the circuit, wherein an effective variable circuit capacitance is obtained by operation of the clamping action.